We have five senses, six some argue (but that's debatable). Meditation in my understanding is trying to minimise inputs into our sensory instruments and allow our mind to focus (info from Discovery Channel). In Zen meditation we allow our mind to wander and explore our subcouscious while disallowing external input to overwhelm our reflective mind.

Maybe o'sensei was doing moving Zen, trying to shut out unneccesary stimuli while trying to concentrate on doing his aikido.